Conventionally, when manufacturing a buffered optical fiber, at least two coating layers made of silicon-based thermosetting type resin or UV curing resin are applied to the circumference of a glass optical fiber having a diameter of approximately 125 μm to form an optical fiber and this circumference is coated with polyvinyl chloride resin, nylon resin, polyester elastomer resin or the like to form a buffered optical fiber. Though it depends on the use, optical fibers having an outer diameter of approximately 250 μm and approximately 400 μm are used most.
One of important characteristics of a buffered optical fiber is a pistoning characteristic. “Pistoning” means a phenomenon that a glass optical fiber inside the buffered optical fiber protrudes from an end face of coating and when the pistoning characteristic is poor, that is, when the amount of pistoning of the glass optical fiber from the end face of coating is large, there are problems with connections such as occurrence of breaking of the glass optical fiber in the connector. A buffered optical fiber used for a laser module or the like in particular is required to have an extremely small amount of glass pistoning.
As a buffered optical fiber with an improved pistoning characteristic, there are proposals such as one characterized in that Young's modulus of secondary coating is 250 MPa or less and primary stripping force of the optical fiber is 100 g/10 mm to 700 g/10 mm (see Patent Document 1) and another one characterized in that a primary coating layer has a two-layer structure of a buffering layer having low Young's modulus and a protective layer having high Young's modulus, Young's modulus of the buffering layer of the primary coating layer is 0.8 MPa or less, breaking strength is 3.0 MPa or more and glass pulling out force is 0.4 to 1.5 N/mm (see Patent Document 2).    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-252388    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-189390